The present invention relates to the field of laser engineering and fiber optics and is industrially applicable in fiber communication systems for pumping optical amplifiers used in wide-band fiber-optical communication systems. Furthermore, the proposed device may be used as a source of radiation in fields where the spectral-selective action of radiation on a substance in the near infrared range is required, in particular in medicine, and also for diagnosis of the environment, in chemistry.
It is well known that the effect of stimulated Raman scattering (the scattering of light on intramolecular oscillations) may be used to amplify optical radiation (see Y. R. Shen, The Principles of Nonlinear Optics, A Wiley-Interscience Publication John Wiley and Sons, New York-Chichester-Brisbane-Toronto-Singapore, (copyright) John Wiley and Sons, Inc., 1984, chapter 10, pp. 141-186.
In the specification of the proposed device the concept of Raman amplification factor in a voluminous sample of the material and in a single-mode fiber with a core and cladding of a predetermined makeup is given substantial consideration.
Here and below the Raman amplification factor GR is understood to mean the Raman amplification factor in a voluminous material, as it is defined, for example, in Y. R. Shen, The Principles of Nonlinear Optics, A Wiley-Interscience Publication John Wiley and Sons, New York-Chichester-Brisbane-Toronto-Singapore, (copyright) John Wiley and Sons, Inc., 1984, chapter 10, pp. 143-146.
The Raman amplification factor in a fiber go is used to describe the amplification properties of fiber in particular. This amplification factor is related to the value GR for the material of the core of the fiber by the relationship go=GR/Aeff, where Aeff is the effective area of the core of the fiber (see, for example, Govind P. Agrawal, Nonlinear Fiber Optics, [Quantum electronicsxe2x80x94principles and applications], Academic Press, Inc., Harcourt Brace Jovanovich, Publishers, Boston-San Diego-New York-Berkley-London-Sydney-Tokyo-Toronto, 1989, Chapter 8, pp. 218-228). The dimension of the factor go is 1/(m W) or dB/(km W). In particular, for a number of communication fibers, the value of go was measured experimentally (V. L. da Silva, J. R. Simpson. Comparison of Raman Efficiencies in Optical Fibers. Conference on Optical Fiber Communications, 1994, OFC""94 Technical Digest, WK13, pp. 136-137, 1994) and was xcx9c5 10xe2x88x924 1/(mW) or, which is the same, xcx9c2.2 dB/(km W).
Constructions of fiber lasers are known, the action of which is based on the effect of stimulated Raman scattering in a fiber, and which are actually converters of the frequency (or wavelength) of optical radiation. In particular, a Raman fiber laser based on a germanosilicate fiber is presented in the paper (S. G. Grubb, T. Strasser, W. Y. Cheung, W. A. Reed, V. Mizhari, T. Erdogan, P. J. Lemaire, A. M. Vengsarkar, D. J. DiGiovanni, D. W. Peckham, B. H. Rockhey. High-Power 1.48 xcexcm Cascaded Raman Laser in Germanosilicate Fibers, Optical Ampl. and Their Appl., Davos, USA, 15-17 June 1995, pp. 197-199).
The five-stage Raman laser presented in this paper is designed to convert optical radiation with a wavelength of xcexo=1.117 xcexcm (and frequency "ugr"o=8950 cmxe2x88x921) into radiation having a wavelength of 1.48 xcexcm (6760 cmxe2x88x921). GeO2 is the main impurity dope in the core of the fiber. The pumping source is an ytterbium laser with a generation wavelength of 1.117 xcexcm. The Raman fiber laser comprises five pairs of distributed Bragg fiber gratings as reflectors for the wavelengths xcex1=1.175 xcexcm ("ugr"1=8511 cmxe2x88x921), xcex2=1.24 xcexcm ("ugr"2=8065 cmxe2x88x921), xcex3=1.31 xcexcm ("ugr"3=7634 cmxe2x88x921), xcex4=1.40 xcexcm ("ugr"4=7143 cmxe2x88x921), xcex5=1.48 xcexcm ("ugr"5=6760 cmxe2x88x921), forming respectively 5 resonators enclosed one in another, which comprise a germanosilicate fiber, for the 1st, 2nd, 3rd, 4th and 5th Stokes components of the Raman scattering in a germanosilicate fiber. When pumping radiation is launched into the aforesaid germanosilicate fiber, amplification of the optical radiation in the frequency range shifted toward the long-wave side relative to the pumping radiation by a value of about 450 cmxe2x88x921 occurs due to the effect of stimulated Raman scattering. The magnitude of the shift and the bandwidth of the amplification are determined by the characteristics of the oscillations of molecules of the fiber material which is used in the Raman laser as the active medium, in this particular case by the properties of germanosilicate glass. When the value of the amplification of the optical radiation at a frequency of "ugr"1 reaches some threshold value, laser generation occurs at that frequency. The 1st stage of the Raman laser works in this way. After initiation of generation at the frequency "ugr"1, the radiation with the frequency "ugr"1 will already serve as a pump for the Raman laser, the resonator of which is tuned to the frequency "ugr"2, and so on to the fifth conversion stage. Thus, each stage of the Raman laser under consideration shifts the generation frequency by a value of xcx9c450 cmxe2x88x921.
A drawback of this laser is the relatively low efficiency of radiation conversion in the 5th Stokes component, which is due to losses in the optical fiber at wavelengths of the Stokes components because of the large number of stages of conversion as a result of the small value of the Raman shift of the radiation frequency in standard fibers, in particular because of the presence of optical resonators in the shortwave range of the spectrum (in this particular case a 1.175 xcexcm resonator), where, as is known, the optical losses in a fiber have a large value as compared with the range of about 1.55 xcexcm.
U.S. Pat. No. 5,323,404, IPC H 01 S 3/30, published Jul. 21, 1994, comprises a disclosure of a similar device. It is noted here that a Raman laser of this type is capable of generating practically any predetermined wavelength due to the large relative width of the Raman amplification band in a germanosilicate fiber. But the device has the same drawback as in the preceding example.
The device most similar to the claimed device is the known Raman fiber laser disclosed in U.S. Pat. No. 5,838,700, IPC H 01 S 3/30, published Nov. 17, 1998, which comprises as the active medium a length of a fiber that comprises an oxide array on the base of SiO2, the makeup of which includes phosphorus oxide, a laser with a generation wavelength of 1.0-1.1 xcexcm as a pumping source, and two refractive index Bragg gratings as distributed reflectors for a wavelength range from 1.20 to 1.28 xcexcm, forming a resonator for the first Stokes component and two more Bragg gratings as distributed reflectors for a wavelength range from 1.46 to 1.50 xcexcm, forming a resonator for the second Stokes component. In accordance with the instant invention, at a pumping radiation wavelength of 1.0-1.1 xcexcm, radiation with wavelengths of 1.2-1.28 xcexcm and 1.46-1.50 xcexcm may be obtained in Raman lasers on a phosphosilicate fiber as a result of only one- or two-stage conversion, respectively. As a result, a higher efficiency of conversion may be achieved than in a Raman laser based on a germanosilicate fiber (which is demonstrated, for example, in the paper of E. M. Dianov, I. A. Bufetov, M. M. Bubnov, A. V. Shubin, S. A. Vasiliev, O. I. Medvedkov, S. L. Semjonov, M. V. Grekov, V. M. Paramonov, A. N. Gur""yanov, V. F. Khopin, D. Varelas, A. Iocco, D. Constantini, H. G. Limberger, R. -P. Salathe, xe2x80x9cCW Highly Efficient 1.24 xcexcm Raman Laser Based on Low-loss Phosphosilicate Fiber,xe2x80x9d OFC""99, Technical Digest Series, Postdeadline Papers, PD-25, 1999). The spectrum of a spontaneous Raman scattering in a phosphosilicate fiber (see, for example, E. M. Dianov, M. V. Grekov, I. A. Bufetov, S. A. Vasiliev, O. I. Medvedkov, V. G. Plotnichenko, V. V. Koltashev, A. V. Belov, M. M. Bubnov, S. L. Semjonov and A. M. Prokhorov, xe2x80x9cCW High Power 1.24 xcexcm and 1.48 xcexcm Raman Lasers Based on Low Loss Phosphosilicate,xe2x80x9d Electron. Lett., 33, 1542, 1997) comprises a line xcx9c50 cmxe2x88x921 wide, the center of which is displaced relative to the pumping line bxcx9c1330 cmxe2x88x921, which corresponds to radiation scattering at relatively high frequency intramolecular oscillations related to phosphorus oxide. This value of the Raman shift of the frequency is xcx9c3 times more than the Raman shift in a germanosilicate fiber. It is just this Raman band that is related to the presence of phosphorus oxide in the fiber that is used in the foregoing example to convert pumping radiation with a wavelength of about 1.06 xcexcm to the ranges of 1.24 xcexcm and 1.48 xcexcm for one and two conversion stages, respectively.
A drawback of the prototype is the use of only the aforesaid narrow spectral Raman band of the Raman spectrum of the phosphosilicate fiber, this being related to the presence of phosphorus oxide in the core material, which does not make it possible in practice to obtain radiation of a predetermined arbitrary wavelength at the output of the Raman laser, even taking into account the possible region of readjustment of the usually used pumping radiation sourcesxe2x80x94ytterbium or neodymium lasers in the range of from 1.0 to 1.12 xcexcm.
In accordance with the foregoing, the object of the present invention is to create a Raman fiber laser ensuring enhancement of the efficiency of conversion of the pumping radiation into radiation of a predetermined arbitrary wavelength in the range of from the pumping wavelength to 1.8 xcexcm.
This result is achieved in that in a Raman fiber laser comprising: a length of a phosphosilicate fiber as the active medium, the makeup of the fiber including SiO2, P2O5; a pumping source of optical radiation at a frequency of "ugr"o; at least two pairs of Bragg gratings as reflectors, wherein each pair of Bragg gratings forms an optical resonator including at least a part of a length of the phosphosilicate fiber, while the frequency "ugr"o of the aforesaid pumping source of optical radiation and frequency of each optical resonator "ugr"i, where i=1, . . . N, are set in such a manner that a portion of the values of the frequency differences xcex94"ugr"1="ugr"i-1xe2x88x92"ugr"1 belongs to the range of 1305-1355 cmxe2x88x921, the Raman amplification in which is due to the presence of P2O5 in a phosphosilicate fiber, while the remaining values of the frequency differences xcex94"ugr"1 belong to the range of 50-560 cmxe2x88x921, the Raman amplification in which is due to the presence of SiO2 in the phosphosilicate fiber, wherein there is at least one value of xcex94"ugr"i in each of the ranges.
Wherein, it is preferable that an additional Bragg grating with a radiation reflection factor at the frequency "ugr"o greater than 50% is disposed at the output of the Raman fiber laser.
Furthermore, the pumping radiation source may be made in the form of a neodymium fiber laser with a frequency "ugr"o selected from the ranges of 8930-9489 cmxe2x88x921 , 10820-10870 cmxe2x88x921 and 7400-7700 cmxe2x88x921.
The pumping radiation source may be made in the form of an ytterbium fiber laser with a frequency "ugr"o selected from the range of 8900-10000 cmxe2x88x921.
In particular the concentration of P2O5 in the core of the aforesaid portion of the phosphosilicate fiber lies within the range of 8-15 mole %.
Furthermore, a compound of at least one chemical element of the group Ge, N, Ga, Al, F, Ti, B, Sn, S, may be additionally introduced into the makeup of the aforesaid length of the phosphosilicate fiber.
Thus, a specific feature of the proposed construction of the Raman laser based on a phosphosilicate fiber is the use of a Raman shift, related to the silicate matrix, in a number of stages of radiation frequency conversion.
The simultaneous use in a multistage Raman laser of at least two substantially different values of Stokes frequency generation shifts in different stages makes it possible, on the one hand, due to the use of stages with a large Stokes shift, to obtain radiation with a wavelength differing from the necessary by less than the value of the large shift (in this case xcx9c1330 cmxe2x88x921) with the minimum necessary number of conversion stages. After that by means of one or two Raman conversion stages with frequency shifts taken from the range of 50-560 cmxe2x88x921, it is possible to obtain the exact value of the predetermined generation wavelength. Additional possibilities in the selection of the final wavelength of radiation conversion are provided by the selection of the pumping radiation wavelength of a neodymium fiber laser (in the range of from 1.055 to 1.10 xcexcm) or an ytterbium fiber laser (from 1.05 to 1.12 xcexcm).
In particular, in order to enhance the degree of use of pumping radiation in the scheme of a Raman laser based on a phosphosilicate fiber, an additional single Bragg grating may be introduced that has a high (greater than 50%) radiation reflection factor at a frequency "ugr"o and is positioned near the output of the Raman laser.
It is important that the Raman amplification factor has approximately equal values in both bands of the Raman spectrum of the phosphosilicate fiber (the value of go for each range (1305-1355) cmxe2x88x921 and (50-560) cmxe2x88x921. In that case the optimum lengths of the fiber for each frequency conversion stage also have approximately equal values. This makes it possible to use one and the same length of the fiber as an active medium with a length close to the optimum length for all of the stages of the Raman laser, both with large (1305-1355) cmxe2x88x921 and with small (50-560) cmxe2x88x921 Raman frequency shift. Therefore, it is preferable to use a phosphosilicate fiber with a concentration of the phosphorus oxide lying within the range of 8-15 mole % as the Raman gain medium.
The use of high frequency and low frequency Raman frequency shifts simultaneously in one Raman laser makes it possible to obtain radiation of any predetermined wavelength in a range between the pumping radiation wavelength and ≈1.8 xcexcm with a small number of Raman conversion stages and with high efficiency. In particular, a Raman laser based on a phosphosilicate fiber with pumping from ytterbium or neodymium fiber lasers makes it possible to obtain with high efficiency (xe2x89xa750%) radiation frequency conversion from a pumping radiation wavelength of xcx9c1000 nm to any wavelength in the range to 1.6 xcexcm with no more than 3 conversion stages.